This invention relates to a spread spectrum phase-locked loop clock generator.
Timing control of many electronic devices, such as microprocessors, is provided by a clock signal. Typically, the clock signal has a frequency of several megahertz. The waveform of the clock signal is approximately trapezoidal in form, and the clock signal normally has energy at its fundamental frequency and at each integer multiple of the fundamental frequency.
The clock signal may be generated by a crystal oscillator. The crystal oscillator has the advantage that its oscillation frequency is stable, so that it is relatively easy to guard against the possibility that drift will result in the fundamental frequency of the clock signal exceeding the clock frequency rating of the microprocessor and leading to a malfunction of the microprocessor.
The crystal oscillator has a very high Q value, which results in the energy of the clock signal being concentrated in a narrow fundamental frequency band and harmonics of that band. The concentration of energy at the high frequencies of the harmonics of the clock frequency can lead to emission of electromagnetic interference (EMI) in excess of that permitted under applicable government regulations.
It is known to reduce the level of electromagnetic interference resulting from a microprocessor clock signal by varying the frequency of the clock signal, so that the energy is not concentrated in a narrow fundamental frequency band and the harmonics of that band. For example, the IMISG501 system clock chip sold by International Microcircuits, Inc. of Milpitas, Calif. employs intentional broadbanding to reduce electromagnetic interference.
On first consideration, it might appear attractive to vary the frequency of the clock signal in accordance with a symmetrical triangular waveform composed of two ramps of equal but opposite slope. However, on closer examination, it becomes apparent that this does not provide the optimum solution because at the positive and negative turning points of the waveform there is a region over which the clock frequency varies only slightly. This results in excessive EMI at the two extremes of the range through which the frequency is varied.